The present invention relates to a power converter which converts alternating current into direct current and/or vice versa and more specifically to a power converter suitable for use in commutatorless motor systems, power frequency conversion systems, and conversion systems for d.c. power transmission.
An example of a conventional power converter will be described with reference to FIG. 1. As shown in FIG. 1, the power converter comprises a converter 3 which converts alternating current into direct current, an inverter 4 which converts direct current into alternating current, a direct-current reactor 5 connected between the converter 3 and the inverter 4 which smoothes d.c. current that flows from the converter to the inverter.
The power converter further comprises a converter control circuit 85 which controls output current of the converter 3 with reference to the phase of a power source 1 and an inverter control circuit 86 which controls the output frequency of the inverter 4 on the basis of the phase of internal induced voltage of a synchronous motor 6.
The converter 3 is formed mainly from thyristors 7 to 18, which constitute arms connected in a bridge configuration.
The inverter 4 is also formed mainly from thyristors 87 to 92 and 93 to 98, which constitute arms connected in a bridge configuration.
The converter 3 and the inverter 4 are power converters of so-called separately excited current type which convert d.c. current smoothed by the d.c. reactor 5 into a.c. current by commutation based on the voltage of the power source 1 or the synchronous motor 6.
Each of the arms of the converter 3 and the inverter 4 comprises a number of series connected thyristors. For the purpose of simplifying description, each arm is described herein as comprising two devices connected in series.
The above arrangement is known as a so-called commutatorless motor system which controls the output current of the converter 3 by the converter control circuit 85 with reference to the phase of the power source 1 and controls the output frequency of the inverter 4 by the inverter control circuit 86 in accordance with the phase of internal induced voltage of the synchronous motor 6.
Next, the operation of the power converter of FIG. 1 will be described with reference to FIG. 2, in which VDC stands for d.c. output voltage of the converter 3, ID stands for current flowing into a d.c. circuit, VDI stands for d.c. input voltage to the inverter 4, IU stands for current supplied from the U phase of the inverter 4 to the U phase of the synchronous motor 6, and IV stands for current supplied from the V phase of the inverter 4 to the V phase of the synchronous motor 6.
In the W phase of the inverter 4 there flows current that is delayed in phase by 120.degree. with respect to IV. VUV is UV line voltage in the inverter 4. The VW line voltage and the WU line voltage in the inverter 4 are delayed in phase by 120.degree. and 240.degree., respectively, with respect to VUV but have similar waveforms.
Here, the commutation process from U phase to V phase in the inverter 4 will be explained by way of example. First, at time just short of t1, the U-phase thyristors 93 and 87 and the Z-phase thyristors 92 and 98 in the inverter 4 are turned ON and thus current flows from the U phase of the inverter 4 through the U and V phases of the synchronous motor 6 into the Z phase of the inverter 4. When, at time t1, the V-phase thyristors 94 and 88 are turned ON, the UV line voltage VUV is shorted by the U-phase thyristors 93 and 87 and the V-phase thyristors 94 and 88 and falls to zero.
At this point, commutation voltage indicated by A due to UV leakage inductance in the synchronous motor 6 is added to the UV line voltage VUV, decreasing the U-phase current IU and increasing the V-phase current IV. Thus, commutation is achieved.
At time t2, the U-phase current IU falls to zero, completing commutation. TQ is torque generated by the synchronous motor 6 and is in proportion to active power injected for the internal induced voltage of the synchronous motor 6. RPW is reactive power injected for the internal induced voltage of the synchronous motor 6.
FIG. 3 shows a circuit diagram illustrating another example of a conventional power converter. In this figure, like reference numerals are used to denote corresponding parts to those in FIG. 1.
In FIG. 3, the a.c. side of the converter 3 is liked with a first power supply 99 through a first transformer 100 to convert alternating current into direct current or vice versa.
The a.c. side of the inverter 4 is liked with a second power supply 101 through a second transformer 102 to convert direct current into alternating current or vice versa.
Leading capacitors 103 are associated with the first power supply 99. Likewise, leading capacitors 104 are associated with the second power supply 101.
In FIG. 3, the converter control circuit and the inverter control circuit are omitted for simplicity.
The above arrangement is known as a frequency conversion system for power system that interchanges electricity between power supply systems different in frequency or a conversion system for d.c. power transmission which converts a.c. power into high d.c. voltage for transmission to a distant place and reconverts the transmitted d.c. voltage into a.c. power.
The operation of the power converter of FIG. 3 will be explained with reference to FIG. 4, which is a waveform diagram for use in explanation of the operation of the converter 3 in FIG. 3. The operation of the inverter 4 remains unchanged from that of the converter 3 and thus the description thereof is omitted herein.
In FIG. 4, p is active power of the converter 3, while Q is reactive power. IV is current supplied from the U phase of the first transformer 100 to the U phase of the converter 3. IU is current supplied from the V phase of the first transformer 100 to the V phase of the converter 3.
In the W phase of the converter 3 there flows current that is delayed in phase by 120.degree. with respect to IV. VUV is UV line voltage in the converter 3. The VW line voltage and the WU line voltage in the converter 3 are delayed in phase by 120.degree. and 240.degree., respectively, with respect to VUV but have similar waveforms.
Here, the commutation process from U phase to V phase in the converter 3 will be explained by way of example.
At time just short of t1, the U-phase thyristors 7 and 10 and the Z-phase thyristors 15 and 18 in the converter 3 are turned ON, so that current flows from the Z phase of the converter 3 through the W and U phases of the first transformer 100 into the U phase of the converter 3. At time t1, a firing pulse is applied to the V-phase thyristors 8 and 11 of the converter 3.
At this point, since the UV line voltage VUV of the first transformer 100 is negative, a forward voltage equal to VUV is applied across the series connection of V-phase thyristors 8 and 11, turning them ON. Thus, the UV line voltage VUV is shorted by the U-phase thyristors 7 and 10 and the V-phase thyristors 8 and 11 and falls to zero.
At this point, a commutation voltage due to leakage inductance of the first transformer 100 is added to the UV line voltage VUV, decreasing the U-phase current IV and increasing the V-phase current IU. Thus, commutation is performed.
At time t2, the U-phase current IU falls to zero, completing commutation. Immediately after time t2, the UV line voltage VUV indicated at A is applied to the U-phase thyristors 7 and 10 ("B").
In the conventional power converters described above, each arm in the converter 3 and the inverter 4 is composed of thyristors. The thyristors, having features that forward voltage in the on state is low enough to allow high current to flow through, switching loss is low, and so on, allow high-efficiency, high-voltage, large-capacity converters to be produced in small size and at low cost. In addition, the thyristors are little subjected to stress at switching time and are therefore very highly reliable.
However, the thyristors have not self-extinguishing capability; therefore, commutation has to depend on a.c. voltage.
That is, as shown in FIG. 2, commutation from U phase to V phase is performed only while the UV line voltage VUV is positive. In the commutatorless motor driving system as shown in FIG. 1, therefore, the torque TQ generated by the synchronous motor 6 contains large ripple components, which may cause vibrations and noise.
In the power frequency conversion system or the d.c. power transmission conversion system as shown in FIG. 3, commutation from U phase to V phase is performed only while the UV line voltage VUV is negative. The thyristors have a turn-off time, during which a reverse voltage has to be applied; otherwise, reignition occurs (see "B" in FIG. 4).
For this reason, commutation must be started turn-off time+commutation time earlier than the time at which the UV line voltage VUV changes from negative to positive (180.degree.) (in general, the margin-of-commutation angle is 30.degree.).
Thus, lagging reactive power is generated as indicated at Q in FIG. 4, which needs the leading capacitors 103 and 104 of large capacity.
Furthermore, the converter 3 and the inverter 4 are operated under poor power factor conditions, providing low utilization of the thyristors.